The purposes of the formation of a chromate conversion coating on the surface of aluminum are to provide corrosion resistance, improve adhesion of coatings and for aesthetic reasons. The conversion coating improves the adhesion of coating layers such as paints, inks, lacquers and plastic coatings. A chromate coating is typically provided by contacting aluminum with an aqueous composition containing hexavalent or trivalent chromium ions, phosphate ions and fluoride ions. Typical chromium or chromate conversion coatings exhibit visible coloration ranging from gold to brown.
Growing concerns exist regarding the pollution effects of chromate and phosphate discharged into rivers and waterways by such processes. Because of the high solubility and the strongly oxidizing character of hexavalent chromium ions, conventional chromate conversion coating processes require extensive waste treatment procedures to control their discharge.
Chromate-free conversion coatings are known in the art. For example, U.S. Pat. No. 4,191,596 which issued to Dollman et al. discloses a composition for coating aluminum which comprises a polyacrylic acid and H.sub.2 ZrF.sub.6, H.sub.2 TiF.sub.6 or H.sub.2 SiF.sub.6. U.S. Pat. No. 4,921,552 which issued to Sander et al. discloses a dried-in-place, non-chromate coating for aluminum. The coating composition consists essentially of H.sub.2 ZrF.sub.6, a water soluble acrylic acid and homopolymers thereof and hydrofluoric acid.
U.S. Pat. No. 4,136,072 which issued to Muro et al., discloses a composition and process for the pretreatment of aluminum surfaces using an aqueous acidic bath containing a stable organic film forming polymer and a soluble titanium compound. U.S. Pat. No. 5,158,622 which issued to Reichgott et al. discloses a dried-in-place conversion coating for metal surfaces such as aluminum and aluminum alloys which employs an aqueous solution of water soluble maleic or acrylic acid/allyl ether copolymers alone or with an acid.
Most non-chromate pretreatments generate transparent coatings on metal surfaces. Furthermore, the lack of chromate makes actual coating weight measurements difficult. The coating weight of a chromate-based coating can be determined relatively easily by chrome X-ray fluorescence or chemical stripping and chemical titration of the coating. Non-chromate coatings may or may not be easily analyzed depending upon the materials present in the coating. For example, anionic polyacrylamide alone or in combination with a nonionic surfactant provides an effective aluminum pretreatment, however, such coatings are not readily analyzed.